1. Field of the Invention
The invention relates to a drive device for a rotary anode of an X-ray tube, including an induction motor whereto an alternating voltage can be applied by means of an inverter, and also including a control unit for controlling the inverter, the switching frequency of the inverter being variable in time, in conformity with a frequency time characteristic, by means of the control unit.
2. Description of Related Art
A drive device of this kind is known, for example for the X-ray tubes of the Philips series "Super-Rotalix". The rotary anode of this known X-ray tube is driven by means of an asynchronous motor which is fed via an inverter. The rotor of the asynchronous motor, being coupled to the rotary anode of the X-ray tube, is accommodated inside the vacuum envelope of the X-ray tube and hence is subject to large temperature fluctuations. In the X-ray mode, i.e. when the electron rays are incident on the rotary anode, the rotor is heated to a temperature as high as 350.degree. C. Outside the X-ray mode, the rotor temperature decreases to substantially room temperature after a sufficiently long cooling down period.
The electrical resistance of the rotor also varies as a function of the fluctuations of the rotor temperature and hence also the electrodynamic properties of the asynchronous motor, notably its starting-up behavior.
In order to ensure uniform starting up of the rotary anode of the X-ray tube, irrespective of the relevant rotor temperature and the rotor resistance resulting therefrom, the known X-ray tube is provided with a temperature monitoring circuit which calculates the temperature of the rotor from the operating characteristics of the X-ray tube on the basis of mathematical models. The calculated temperature is applied to a control unit which controls the switching frequency of the inverter. During starting up of the asynchronous motor, the switching frequency of the inverter is controlled in a temperature-dependent manner by means of the control unit, i.e. in conformity with the relevant temperature of the rotor, and use is made of different frequency time characteristics, each of which ensures that a minimum number of revolutions is reached after an adjustable starting-up period.
The construction of such a temperature monitoring circuit is very complex and intricate.